Evidence from other laboratories suggests that the synthesis of ribosomal RNA is under a complicated system of controls, both negative and positive, and we have been trying to isolate conditional mutants with impaired rRNA synthesis. Since the spc and lin genes bracket the major rRNA cluster, we have used mutagenized transducing bacteriophage, grown on B. subtilis spc, lin donors, and have selected for the antibiotic resistance markers or for the CysA plus gene, a closely linked marker. The transductants are then analyzed for inability to grow at elevated temperatures (over 48 degrees centigrade). When such ts mutants are obtained, they are tested for turn-off of protein and RNA synthesis at the restrictive temperature. Thus far, we have isolated several protein synthesis mutants, linked to spc, but have not, as yet, found a mutant which shows a specific inhibition of rRNA synthesis, though we have found some which may fit this description. ts mutations in this region are rare, approximately 1 per 10 to 20,000 recombinants analyzed.